Propylene copolymers containing from 0.1 to 10 by weight of ethylene, in which the comonomer is randomly distributed in the polypropylene chain, are known as random propylene copolymers. Compared with propylene homopolymers, these copolymers have molecular structures that are modified by the presence of the comonomer, leading to a substantially lower degree of crystallinity. As a result, random copolymers have a lower melting temperature with respect to propylene homopolymers, as well as lower sealing temperatures and moduli of elasticity.
However, the introduction of the comonomer into the polypropylene chain can leads to significant increases in the fraction of polymer which is soluble in xylene at 25° C., with the soluble polymer portion comprising low molecular weight chains and percentages of comonomer which are higher than the average content of comonomer as calculated for the whole polymer. The amount of xylene soluble fraction generally increases as the content of comonomer in the copolymer increases and, beyond defined limits, precludes the use of the copolymers in certain sectors, for example in the preparation of films for food applications, unless recourse is made by the elimination of the xylene soluble fraction. The presence of relevant amounts of the xylene soluble fractions decreases the flowability of the polymer granules, thereby making operations such as discharging and transferring the polymer difficult and giving rise to operational problems in the polymerization plant. Moreover, the presence of the xylene soluble fractions in significant amounts leads, over time, to phenomena such as the deterioration of the film's optical properties and organoleptic properties related to the migration of these fractions to the surface (known as blooming).
It is known in the relevant art that random propylene copolymers with improved comonomer distributions may be obtained using single-site catalysts.
For instance, WIPO Pat. App. Pub. No. WO 2007/45600 relates to random propylene copolymers having high melt flow rates for injection molding and melt blowing applications.
The copolymers described therein have a melt flow rate ranging from 90 to 3000 g/10 min and a distribution of molecular weight of lower than 4. This material is obtained by using metallocene-based catalyst system. Even if the xylene solubles of this material is less than 2.2, the other features of the copolymers, such as the high melt flow rate and the narrow distribution of molecular weight, reduces the effectiveness of these materials for the production of cast films.
WIPO Pat. App. Pub. No. WO 2006/120190 relates to random propylene/ethylene copolymers having an ethylene content ranging from 4.5 to 7 wt % and a Mw/Mn of lower than 4. The copolymers described in this document shows very low levels of xylene solubles after visbreaking; however, the xylene solubles of the ex reactor polymers are comparatively high.
U.S. Pat. No. 6,365,685 (and WIPO Pat. App. Pub. No. WO 97/31954) relates to propylene random copolymers obtained by using a phthalate based catalyst in combination with certain 1,3-diethers as external donors. The random propylene polymers described therein are improved with respect to those obtained with the same phthalate-based ZN catalysts using silanes as external donors. However, the properties of the random copolymers still need to be improved, particularly as the xylene solubles content reported in the patent is determined by a method comprising dissolving the whole sample at the xylene boiling point, lowering the temperature of the solution to 0° C. and allowing the temperature to increase to 25° C. This method as shown in the Comparative Examples of the document gives rise to lower values of xylene solubles.